This invention relates to an alcohol concentration detecting apparatus and, more particularly, to an apparatus for detecting alcohol concentration in liquid fuel by measuring the refraction ratio of the fuel.
In recent years, a mixture of gasoline and alcohol has been increasingly used as fuel for motor vehicles in order to reduce the petroleum consumption. In order to effectively use such alcohol containing gasoline as automotive fuel, the alcohol concentration within the fuel is detected and an actuator such as a fuel injection valve is controlled to regulate the air-to-fuel ratio, the ignition timing and the like in accordance with the alcohol concentration.
Examples of the alcohol concentration detector of this type are disclosed in Japanese Patent Laid-Open No. 57-51920, Japanese Patent Laid-Open No. 58-129235 and Japanese Patent Publication No. 57-12461.
FIG. 11 is a schematic diagram illustrating a fuel control system employing a conventional alcohol concentration detector, in which reference character A indicates an alcohol concentration detector, reference numeral 137 indicates an engine, 136 indicates a fuel injection valve, 122 indicates a fuel tank and 123 indicates a fuel pump.
The fuel within the tank 122 is pumped up by the fuel pump 123 to be fed through a fuel supply pipe 124, a high pressure filter 125, the alcohol concentration detector A, a fuel distributor pipe manifold or pipe 126 to the fuel injection valve 136, where the fuel is injected into the engine 137.
Reference numeral 127 is a fuel pressure regulator 128 is a fuel return pipe, 129 is a air-to-fuel ratio sensor, 130 is an ignition plug, 131 is an engine revolution sensor, 132 is a suction pressure sensor, 133 is a throttle, 134 is an air cleaner and 135 is a control unit.
The control unit 135 receives a signal from the alcohol concentration detector A, a signal from the air-to-fuel ratio sensor and signals from the engine revolution sensor 131 and the suction air pressure sensor 132, in accordance of which the fuel injection valve 136, the ignition plug 130 and the like are driven.
When a fuel to which alcohol is mixed is supplied to the fuel tank 122 and the engine 137 is started, the fuel with alcohol is pumped by the fuel pump 123 to the alcohol concentration detector A, through the fuel pipe 124 and the high pressure filter 125, where the alcohol concentration is measured.
Then the fuel flows into the fuel distribution pipe 126, from where one part of the fuel is supplied to the engine 137 through the fuel injection valve 136 and the other part of the fuel is returned to the fuel tank 122 through the fuel pressure regulator 127 and the fuel return pipe 128.
The fuel pressure regulator always maintains the pressure of the fuel up to the fuel distributor pipe 126 to a constant value.
When the signal indicative of the alcohol content as measured by the alcohol concentration detector A is supplied to the control unit 135, the control unit determines the engine operating state on the basis of the signals from the engine revolution sensor 131, the suction pressure sensor 132 and the like to control the valve open period of the fuel injection valve 136, whereby the fuel amount to be supplied to the engine 137 is changed so that the air-to-fuel ratio varies. The air-to-fuel ratio is detected by the air-to-fuel sensor 129 so that it is feed back-controlled to a target value corresponding to the engine operating state. The control unit also controls the ignition timing of the ignition plug 130 in accordance with the engine operating state.
FIG. 12 illustrates one example of a conventional alcohol concentration detector A, where reference numeral 111 indicates a cylindrical transparent body, 112 indicates a case, 113 indicates a fuel seal between the cylindrical transparent body and the case, 114 indicates a light-emitting element such as LED, 115 indicates a light-receiving element such as a photo diode, 116 indicates a fuel chamber, 116a indicates a fuel inlet, 116b indicates a fuel outlet, 117a indicates a total reflection light beam, 117b indicates a refraction light beam, and reference numeral 100 indicates a detection circuit for driving the light emitting element 114 and measuring the amount of received light at the light-receiving element 115.
The cylindrical transparent body 111 has a cylindrical circumferential surface in contact with the fuel within the fuel chamber. The light emitted from the light-emitting element 114 impinges at the outer circumference surface of the cylindrical transparent body 111 or at the interface between the transparent body and the fuel. At this time, due to the difference between the refraction index Nf of the fuel and the refraction index Nd of the cylindrical transparent body, a light beam having an angle of incidence equal to or larger than the total reflection angle .phi.=arcSIN (Nf/Nd) is totally reflected as indicated as the total reflection light beam 117a to reach the light-receiving element 115, and a light beam having an angle of incidence smaller than the total reflection angle .phi. is allowed to enter into the fuel as the refraction light beam 117b. Therefore, the light-receiving element receives only the total reflection light beam 117a which is the light beam having an angle of incidence equal to or larger than the total reflection angle .phi..
When the alcohol concentration in the fuel varies, the refraction index Nf of the fuel varies and the angle of total eflection .phi. varies accordingly, so that the amount of light that reaches the light-receiving element 5 varies. Then, the alcohol concentration of the fuel can be determined by measuring the output of the light-receiving element.
In the conventional alcohol concentration detecting apparatus constructed as above described, the amount of light emitted from the light-emitting element 114, the sensitivity to light and the peak sensitivity frequency of the light-receiving element 115 are changed in response to the temperature change. Therefore, when the temperature of the alcohol concentration detecting apparatus rises due to the engine heat and the heated fuel, the output of the detecting apparatus varies, so that the alcohol concentration of the fuel cannot precisely be determined.
Also, since the total reflection angle .phi. must be relatively large because of the limitation on the refraction index Nd of the cylindrical transparent body, the cylindrical transparent body and therefore the entire alcohol concentration detecting apparatus cannot be made compact.
Further, since the alcohol concentration detecting apparatus must be mounted at a position remote from the engine for the reasons discussed above, it is impossible to immediately, without delay, detect the alcohol concentration of the fuel that is to be injected from the fuel injection valve 136 at the time of starting of the engine, for example.
Particularly, when the engine is to be started immediately after a fuel of a different alcohol concentration has been refilled, there may be a mode of starting operation in which the fuel within the fuel distribution pipe 126 has a different alcohol concentration as compared to that of the fuel within the fuel chamber 116 of the alcohol concentration detector A. In the worst case, it is possible that the engine cannot be started.